Pete, you know more about electronics than I do.
I have a Speeduino, its Arduino based. Uses the Mega2560. Plus a dedicated shield. Its for ignition and fuel injection.
I have no idea how 123 works, however ...
Regarding, your 2 wire cops, you may just be able to add a resistor in series. Again, however! You have 2 coils being (im guessing) supplied from one channel from the 123. So your coils are going to be wired in parallel. If I recall ohms law correctly, the resistance of the coils will be reduced further, I think halved in this case. Look it up.
Therefore, the resistor you add in series Would need to compensate for it. It think the rationale behind the correct resistance is to ensure you dont pull to many amps and overload the traces in the 123.Other useful things to reseach are
Bosch 211
Its basiclly an array of transistors that is specifically designed for switching on Ignition coils. Its 4 channel.
Another wayits to optically isolate the coils, though you still need your required resistance. Again, its a transistor, but this time the coil circuit is completely isolated from the 123 circuit. Its all in one chip, but the switch is operated by light, so the circuits never touch.
You may just need a small resistor in line though.
